1. Field of the Invention
The present invention relates to a method of manufacturing a multilayer ceramic substrate, and more particularly, to a hard-to-sinter constraining green sheet utilized for manufacturing a low-temperature co-fired substrate by constrained sintering, and a multilayer ceramic substrate using the same.
2. Description of the Related Art
In general, a multilayer ceramic substrate using glass-ceramics ensures implementation of a three-dimensional inter-layer circuit and formation of a cavity. This allows devices with various functions to be embedded in the multi-layer ceramic substrate, with high flexibility in design.
Accordingly, in the market of smaller and higher-performing high-frequency parts, the multilayer ceramic substrate is increasingly utilized. A multi-layer ceramic substrate in an incipient stage has been manufactured by forming a circuit pattern and a via on a ceramic green sheet as a conductive electrode, arranging and stacking a plurality of the green sheet to a desired thickness according to design. In this process, the ceramic substrate shrinks in volume by about 35 to 50%. Particularly, the ceramic substrate shrinks about 12 to 17% in horizontal and vertical lengths, respectively in a transverse direction. This transverse shrinkage can be hardly controlled uniformly. The transverse shrinkage involves an error of 0.5% occurs in respective manufacturing stages and an identical manufacturing stage as well.
With the multilayer ceramic substrate more complicated and precise, inner patterns and via structures have less margin in design and thus constrained sintering is required to suppress transverse shrinkage of the multilayer ceramic substrate.
To this end, a hard-to-sinter flexible green sheet which is not sintered at a sintering temperature of the ceramic substrate material, is bonded to at least one of two surfaces of the multilayer ceramic substrate in order to suppress shrinkage of the multilayer ceramic substrate in a x-y direction. Notably, a load is applied onto the multilayer ceramic substrate to prevent the substrate from being warped during sintering. Here, the multilayer ceramic substrate may experience lack of passages for de-binding in the process of sintering, thereby degrading sintering characteristics. Moreover, a sintered ceramic stacked body may have great residual carbon content, which can undermine the reliability of the ceramic substrate.
Therefore, in order to impart a sufficient constraining force to the constraining green sheet, constrained material and process are required to allow the constraining green sheet is solidly bonded to the ceramic substrate and de-bound easily during sintering.
Japanese Patent Laid-open Publication No. hei 7-30253 discloses a conventional technology for de-binding. Under this technology, even when a constraining green sheet is employed, de-binding is relatively easily assured. Specifically, a hole is perforated in the constraining green sheet to ensure sufficient de-binding of an inner ceramic substrate and a resin which is more easily thermally decomposable than an organic binder included in a non-sintered ceramic stacked body is filled in the hole. However, this technology entails an additional burdensome process of perforating the hole in the constraining layer and deformability of a sintered body due to the hole.
Moreover, Korean Patent Publication No. 2002-0090296 discloses a technology in which an organic binder having an initial thermal decomposition temperature lower than an organic binder of a green sheet for a sintered body is employed in a constraining green sheet to remove the binder of the constraining green sheet and then the binder of the green layer of the sintered body is released easily through a passage generated thereby.
However, to maximize a constraining force of the constraining green sheet, a powder of the constraining layer should be pulverized and added in a higher amount to maximally increase a contact point between the constraining layer and the ceramic stacked body. This however may not ensure pores to be sufficiently formed inside the constraining green sheet. Unless pores are formed sufficiently, the binder decomposed or burned from the ceramic stacked body can hardly be released outward from hundreds of microns of the green sheet through pores inside the constraining green sheet, even though an organic material of the constraining green sheets is decomposed first. This does not yield sufficient de-binding effect.
Furthermore, an alternative technology is disclosed in Japanese Laid-open Publication No. 2006-173456. Under the technology, as shown in FIG. 1, a volume content of an organic binder 14 and an inorganic powder particle 12 of a constraining green sheet 15 is greater on an area around a free surface 15b than on an area around a contact surface 15a with a multilayer ceramic substrate 11. That is, the organic material contents differ between the contact surface and the free surface to enhance a bonding force between a ceramic substrate and a constraining layer and also to facilitate de-binding toward the free surface of the constraining layer with a greater number of pores.
However, since density gradients of components are formed through precipitation inside the constraining green sheet 15 using doctor blading, it is very hard to attain reproducibility of appropriate thickness and volume contents for respective areas. Moreover, under this technology, powder particles are easily precipitated onto the bottom when the constraining green sheet is formed, thereby requiring an inorganic powder with greater particle size, for example, twice greater than particles of the ceramic substrate to reduce an organic binder amount on the bottom. This renders it hard to obtain a sufficient contact point between the green sheet and the ceramic substrate. Furthermore, this hardly increases a capillary force for moving the organic binder from the ceramic substrate to the constraining green sheet.